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139 



form before any other part. The central portion of the nervous 

 system sometimes attains a considerable degree of developement, 

 although it be exceedingly minute ; thus an instance has been met 

 with in which the developement of this part had reached a stage 

 scarcely inferior to that in another instance, in which the corre- 

 sponding part measured more than ten times the length. 



There does not occur in the mammiferous ovum any such pheno- 

 menon as the "splitting" of a membrane into the so-called "se- 

 rous, vascular, and mucous laminae." Rathke had already found 

 that parts previously supposed by Baer and others to be formed by 

 the so-called "germinal membrane," really originate independently 

 of it : these parts are the ribs, pelvic bones, and the muscles of the 

 thorax and abdomen, which according to Rathke arise in a part pro- 

 ceeding out of the " primitive trace" itself. Reichert had previously 

 discovered that the part originating the lower jaw and hyoid bone 

 " grows out of the primitive trace." The author beginning with an 

 earlier period goes farther than these observers, and shows that 

 the so-called " primitive trace" it«elf does not arise in the substance 

 of a membrane, but presents a comparatively advanced stage of the 

 object above described as the true germ. Hence the author suggests, 

 there is no structure entitled to be denominated the " germinal 

 membrane." 



The most important of the foregoing facts respecting the deve- 

 lopement of the mammiferous ovum, however opposed they may be 

 to received opinions, are in accordance with, and may even explain, 

 many observations which have been made on the developement of 

 other animals as recorded in the delineations of preceding observers. 

 If in the ovum of the bird the germinal vesicle in like manner re- 

 turns to the centre of the yelk, the canal and cavity known to exist 

 in the yelk of that ovum might be thus explained. The ovum may 

 pass through at least one -and- twenty stages of developement, and 

 contain, besides the embryo, four membranes, one of which has two 

 laminse, before it has itself attained the diameter of half a line, a fifth 

 membrane having disappeared by liquefaction within the ovum. 



The size of the minute ovum in the Fallopian tube and uterus 

 affords no criterion of the degree of its developement ; nor do any 

 two parts of the minute ovum, in their developement, necessarily 

 keep pace with one another. 



The proportion of ova met with in these researches, which seemed 

 to be abortive, has amounted to nearly one in. eight. Sometimes 

 two yelk-balls exist in the same ovum. With slight pressure, the 

 ovum, originally globular, becomes elliptical. Its tendency to as- 

 sume the latter form exists especially in the chorion, and seems to 

 be in proportion to its size. 



The author has discovered that when the germinal vesicle is first 

 seen it is closely invested by an extremely delicate membrane. This 

 membrane subsequently expanding is that in which the yelk is 

 formed. He has traced the chorion from stage to stage up to the 

 period when it becomes villous, and shows that it is not, as he for- 

 merly supposed, the thick transparent membrane itself of the ovarian 



